System for evaluating probing networks

ABSTRACT

An interconnect assembly for evaluating a probe measurement network includes a base, respective inner and outer probing areas in mutually coplanar relationship on the upper face of the base, a reference junction, and a high-frequency transmission structure connecting the probing areas and the reference junction so that high-frequency signals can be uniformly transferred therebetween. A preferred method for evaluating the signal channels of the network includes connecting a reference unit to the reference junction and successively positioning each device-probing end that corresponds to a signal channel of interest on the inner probing area.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of patent application Ser. No. 10/951,917, filedSep. 28, 2004, now U.S. Patent No. ______, which is a continuation ofpatent application Ser. No. 10/459,259, filed Jun. 11, 2003, now U.S.Pat. No. 6,803,77; which is a continuation of patent application Ser.No. 09/611,806, filed Jul. 7, 2000, now U.S. Pat. No. 6,608,496, grantedAug. 19, 2003; which is a continuation of patent application Ser. No.09/359,989 filed Jul. 22, 1999, now U.S. Pat. No. 6,130,544, grantedOct. 10, 2000; which is a continuation of patent application Ser. No.09/175,062, now U.S. Pat. No. 5,973,505 granted Oct. 26, 1999; which isa continuation of U.S. Pat. No. 5,869,975 granted Feb. 9, 1999; which isa division of U.S. Pat. No. 5,659,255 granted Aug. 19, 1997; and whichis a division of U.S. Pat. No. 5,561,377 granted Oct. 1, 1996.

BACKGROUND OF THE INVENTION

The present invention relates to a system for high-frequency evaluationof probe measurement networks and, in particular, to a system foraccurately evaluating the signal conditions existing in such networkseven in those ones of such networks, for example, that are of amultichannel type in which each channel communicates through a separatedevice-probing end and even in those ones of such multichannel networks,for example, that have their device-probing ends crowded together in ahigh-density coplanar probing array as suitable for the measurement ofintegrated circuits or other microelectronic devices.

FIG. 1 shows a probe station 20 that includes a multichannel measurementnetwork 21 of a type suitable for measuring high-frequencymicroelectronic devices at the wafer level. A probe station of this typeis manufactured, for example, by Cascade Microtech, Inc. of Beaverton,Ore. and sold under the trade name SUMMIT 10000. The various devices 24,the characteristics of which are to be measured by the network, areformed on the surface of a wafer 22 in isolation from each other. Anenlarged schematic plan view of an individual device 24 is shown in FIG.2. The surface of each device includes a predetermined pattern ofbonding pads 26 that provide points of connection to the respectiveelectrical components (not shown) formed on the central area of eachdevice. The size of each bonding pad is exaggerated for ease ofillustration in FIG. 2, but it will be recognized by one of ordinaryskill in the art that there will typically be hundreds of bonding padsin the rectangular arrangement shown, each of a size that is barelyvisible to the eye without magnification. If a hybrid device instead ofa flat wafer is being tested, then the individual devices can rise todifferent heights above the plane of the hybrid device's upper surface.

As depicted in FIG. 1, to facilitate high-frequency measurement of eachdevice 24, a typical probe station 20 includes a wafer-receiving tableor chuck 28 for supporting the wafer 22. The probe measurement network21 of the station includes a probing assembly 30 which, as shown, cantake the form of a probe card with a multiconductor probe tip array fordelivering signals to, and receiving signals from, the respectivebonding pads of each individual device. One common type of probe cardstructure, as depicted, includes an open-centered rectangular-shapedframe 32 with numerous needle-like probe tips 34 that downwardlyconverge toward the open center of the frame. The end portion of eachtip is bent at a predetermined angle so that the lower extremities ordevice-probing ends of the tips, which typically have been blunted bylapping to form a coplanar array, are suitably arranged for one-to-onecontact with the bonding pads 26 provided on each respectivemicroelectronic device. The measurement signals provided by the networkare generated within and monitored by a multichannel test instrument 36,which is connected to the probe card via a suitable multiconductor cable38. The probe station also includes an X-Y-Z positioner (e.g.,controlled by three separate micrometer knobs 40 a, b, c) for permittingfine adjustments in the relative positions of the probe card 30 and theselected device-under-test.

The individual elements that make up a probe measurement network cantake forms other than those shown in FIG. 1. For example, depending onthe particular requirements of the devices to be measured, the probingassembly can take the form of a multiconductor coplanar waveguide asshown in Strid et al. U.S. Pat. No. 4,827,211 or Eddison et al. UKPatent No. 2,197,081. Alternatively, the assembly can take the form ofan encapsulated-tip probe card as shown in Higgins et al. U.S. Pat. No.4,566,184, or a multiplane probe card as shown in Sorna et al. U.S. Pat.No. 5,144,228, or a dual-function probe card in which the probe card notonly probes but also supports the downturned wafer, as shown in Kwon etal. U.S. Pat. No. 5,070,297. Use of this last card structure, however,is limited to the probing of flat wafers or other device configurationsin which all devices are of the same height.

Before using a probing station or other probing system to measure thehigh-frequency performance of individual devices, such as those formedon a wafer, it is desirable to first accurately evaluate the signalconditions that are actually present in the measurement network of thesystem with reference, in particular, to the device-probing ends of thenetwork.

For example, with respect to a probing system of the type shown in FIG.1, in order to accurately calibrate the source or incoming channels ofthe system's measurement network, preferably measurements are made ofthe respective signals that are generated by the various sourcing unitsof the test instrument 36 in order to reveal how these signals actuallyappear in relation to each other when they arrive at the device-probingends that correspond to the respective source channels, since thesignals that actually enter the input pads of each device come directlyfrom these ends. Conversely, in order to accurately calibrate the senseor outgoing channels of the probing network, preferably the respectivesignal conditions that are indicated by the various sensing units of thetest instrument 36 are observed when reference signals of identical orotherwise relatively known condition are conveyed to the device-probingends that correspond to the respective sense channels, since the signalsthat actually exit the output pads of each device go directly to theseends. Should any channel-to-channel differences be found to exist in thenetwork, these differences can be compensated for so that the testinstrument will only respond to those differences which actually arisefrom the different input/output characteristics of thedevice-under-test.

Typically it is difficult, however, to make comparatively accuratehigh-frequency measurements in reference to the extreme ends of aprobing assembly where the ends have been arranged for the measurementof planar microelectronic devices because of the reduced size and theclosely crowded arrangement of such ends. This is particularly so whenthe probing assembly is of the card-like type 30 shown in FIG. 1, due tothe inherent fragility of the needle-like tips 34 that are part of suchan assembly.

The reason for this difficulty can be better understood in reference toFIG. 3, which shows one common type of interconnect assembly that hasbeen used to evaluate probe measurement systems of the type shown inFIG. 1. This assembly includes a signal probe 42 having a single pointedtransmission end 44, which probe is connected, via a cable, to thesensing unit, for example, of a test instrument. This instrument caneither be the same as that instrument 36 which provides the sourcingunits for the probe measurement network or, as shown, can be an entirelyseparate instrument 46. Viewing FIGS. 1 and 3 together, when the pointedend of the signal probe is being repositioned from one tip 34 toanother, normally it is necessary to move the relatively stiff end ofthe probe slowly and deliberately in order to avoid damaging thedelicate needle-like tips, so that a relatively long period of time isneeded in order to complete evaluation in relation to all the tips.Additionally, this type of probe has poor high-frequency measurementstability in moderately noisy test environments. Even moresignificantly, because the extreme ends of the needle-like tips 34 onthe probe card are too thin and delicate to be probed directly,probe-to-probe contact between the pointed transmission end 44 of thesignal probe and each needle-like tip of the probe card must occurfurther up nearer to the base of each tip. This introduces, for example,a phase offset of indeterminate amount between the signal that is beingmeasured by the signal probe and the signal as it will actually appearin relation to the bonding pads 26 (FIG. 2) of each device. The degreeof this offset, moreover, will generally vary in an arbitrary mannerbetween the different tips, since the pointed end of the probe willnormally be placed into contact with the different tips at somewhatdifferent positions along their respective lengths. Using this type ofcalibration assembly, then, it is difficult, if not impossible, toaccurately evaluate the relationships of the different signals thatactually exit the various device-probing ends of the needle-like tips 34and hence it is difficult, if not impossible, to normalize these signalsor to otherwise calibrate the network so as to permit accurate devicemeasurement.

An alternative approach to probe network evaluation would be to use oneor more of the device-probing ends that are included on the probe carditself, instead of a separate signal probe, for establishing thereference channel back to the original test instrument. In accordancewith this approach, a different form of interconnect assembly would beused. This assembly might include a plurality of conductive paths, suchas those defined by traces formed on a substrate, where the arrangementof the paths would be such that each device-probing end relative towhich evaluation is to be conducted would be connected to one of theends being used to establish the reference channel via a “through”channel formed by one or more of the paths.

However, through channels of this type would constitute less thanperfect transmission lines and, to the extent that the majority of theends are to be evaluated in this manner, these through channels wouldneed to be of different lengths to accommodate such measurement. Hence,even when the same source or sense channel is being evaluated under thisapproach, the measured value of signal condition in the channel willappear to change depending on which through channel of the assembly isbeing used for making the observation. Moreover, since a typical probecard for wafer-level testing has hundreds of probe ends convergingwithin an area less than one-half inch on each side, and since there canbe cross-coupling of signals between closely adjacent paths as well asdistortion caused by the presence of extraneous radiation in themeasurement environment, a suitable physical layout that could provide,for example, adequate high-frequency signal isolation for each path isnot readily apparent.

Although its use is limited to a probe card of quite different type thanthat shown in FIG. 1, another type of high-speed interconnect assemblywhich uses a signal probe for evaluating probing networks is describedin J. Tompkins, “Evaluating High Speed AC Testers,” IBM TechnicalDisclosure Bulletin, Vol. 13, No. 7, pp 1807-1808 (December 1970). As inKwon et al., in Tompkins it is the probe card itself that providessupport for the device-under-test, that is, the device is turned over sothat its bonding pads rest upon a plurality of slightly-raised roundedprobing ends included on the upper side of the card. As in Kwon et al.,this mounting method forecloses the testing of hybrid devices in whichcomponents of different height are mounted on the face of the device. Anadditional disadvantage of the Tompkin's probing network is the poorlyregulated interconductor spacing in the lead-in cable to the card, whichcan result in signal instability at higher frequencies. In any event, toevaluate the signals that are present in the network in reference to therounded probing ends on the card, the interconnect assembly of Tompkinsincludes a two-prong signal probe together with a sheet-like dielectricmember which is placed in a predefined position over thedevice-supporting or upper side of the probe card. Uniformly-spacedholes are formed through the dielectric member and serve as guidechannels for guiding the first prong of the signal probe into tip-to-tipcontact with the various rounded probing ends on the card. At the sametime, a shorter second prong of the signal probe automaticallyestablishes contact with a conductive ground plane which is formed onthe upper side of the dielectric member and which surrounds each hole onthat member.

There are significant difficulties with the type of evaluation approachjust described, however, because the pointed end formed on the firstprong of the signal probe can, over time, wear down the rounded ends ofthe probe card so that these rounded ends eventually lose their capacityto establish simultaneous electrical contact with the planar pads of thedevice-under-test. Furthermore, this measurement approach does notpermit, while device testing is in progress, quick evaluation of signalcondition with respect to a particular probing end of the card, becausethe first prong of the probe normally cannot be applied to any of theends of the card until after the device has been carefully lifted offthe card and removed to a safe static-free location.

Another approach to evaluating the measurement network of a probingsystem employs an impedance standard substrate of the type described,for example, in Carlton, et al., U.S. Pat. No. 4,994,737. An impedancestandard substrate comprises a substrate on which there are knownimpedance standards, which standards are suitably configured forsimultaneous probing by the device-probing ends of the network. Thestandards can include, for example, an open circuit transmission lineelement formed by a pair of spaced-apart pads. Unlike the evaluationmethods thus far described, no separate reference channel is provided toreceive each signal as each signal exits the tip end of a respectiveincoming channel. Instead, the impedance standard on the substrate isused for reflecting the incoming signal so that the signal istransformed at the tip to an outgoing signal which then travels back tothe test instrument through its original signal channel. The electricalcharacteristics of the corresponding signal channel can then be analyzedfrom measurements taken at the test instrument using time-domainreflectometry.

However, in a multichannel network, the differences which exist betweenthe incoming signals at the device-probing ends of the various incomingchannels are a function not only of the differences which exist in therespective circuit characteristics of those channels (i.e., thedifferences in the relative conditions for the signals) but are also afunction of the differences which exist in the signals themselves fromthe moment that each is first generated within a respective sourcingunit of the test instrument (i.e., the differences in the respectiveconditions of the signals). Because the type of evaluation that is madewith an impedance standard substrate only detects differences of theformer sort and not of the latter, this type of approach, at least byitself, cannot be used to fully evaluate the differences in the incomingsignals in reference to the device-probing ends of the measurementnetwork. Conversely, the differences in the signal conditions that areindicated by the various sensing units of the test instrument, even whenreference outgoing signals of identical condition are presented to thedevice-probing ends of the corresponding sense channels, are notobservable using the impedance standard substrate approach. Thus, thisapproach does not permit the different signal conditions of amultichannel probe measurement network to be fully characterized andcompensated for so as to allow accurate device measurement. It may alsobe noted that expensive processing is normally needed in order toproperly evaluate time-domain reflectometry measurements, because thesignal which is evaluated in these types of measurements is prone tosignificant cumulative distortion due to partial reflections occurringalong the channel, conductor losses, frequency dispersion and so on.

One type of probe card evaluation system which is unsuitable forhigh-frequency measurements but which can be used in relation to anarray of probe tips for measuring certain low frequency or DCcharacteristics is sold by Applied Precision, Inc., of Mercer Island,Wash., under the trade name CHECKPOINTJ. The design of this system ispatented in Stewart et al., U.S. Pat. No. 4,918,374, and a similarsystem is apparently made by Integrated Technology Corporation of Tempe,Ariz., under the trade name PROBILT PB500AJ. As described in Stewart,the evaluating system has its own probe card holder. The probe card istransferred to this holder so that the probe card can be held in apredetermined position above a square-shaped checkplate, the upper sideof which is divided into four quadrants. In one characteristicconstruction, at least one of the quadrants contains a narrow conductivestrip extending in either an X or Y reference direction. To determinethe X position of a particular tip, for example, the Y directional stripis moved by incremental movements of the underlying checkplate in the Xdirection toward the tip until a continuity reading between the Ydirectional strip and the tip reveals the precise X position of that tiprelative to the checkplate's original position and hence relative to thecard. In order to determine the positions of several tips at the sametime, in a second construction, one of the quadrants contains a numberof spaced-apart parallel strips that are each wired out to a separateterminal on the sides of the checkplate, thereby making it possible todiscern, for purposes of positional verification, which strip is incontact with which tip.

In order to determine the respective positions of two tips that havebeen electrically tied together at some point up from their ends, yet athird construction is used in Stewart, since under the first twoconstructions there can apparently be some difficulty in determiningvisually which particular tip of the two that are tied together isactually in contact with a strip when continuity is detected. In thisthird construction, one of the quadrants contains a solitary conductivedot of sufficient smallness that only one probe tip at a time can beplaced on the dot, thereby enabling the position of each tip to bedetermined in consecutive sequence. In order to get a proper continuityreading, any other conductor on the checkplate besides the dot isconfined to another quadrant of the checkplate. Hence, any other tipthat might be tied to the tip under test, including a tip on theopposite side of the card, cannot come into contact with anotherconductor as the tip under test approaches the dot, which wouldconfusingly produce the same reading as if the tip under test hadachieved contact with the dot. For apparently similar reasons, theconductive dot is wired out to a terminal that is separate from theterminal of any conductor in the other quadrants.

From the foregoing description of the Stewart evaluation system, it willbe recognized that the principal use of this system is to preciselylocate the relative positions of the device-probing ends of themeasurement network. Although it might be possible to upgrade theStewart system to permit the evaluation of certain lower frequencycharacteristics (such as by adding, perhaps, a lumped capacitor dividernetwork to the Stewart system to measure low-frequency capacitiveeffects), its structure is wholly inadequate for higher frequencymeasurements, such as those ranging above 50 MHZ.

For example, to the extent that the conductor arrangement in Stewartassumes the form of several parallel strips in closely spacedrelationship to each other, if the signal condition in any channel isevaluated via one of these strips, it can appear to vary depending onwhich strip is used (given that the electrical length between each stripand its corresponding terminal varies from strip-to-strip), on whereexactly the device-probing end of the channel is placed in relation tothe elongate strip, and on what types of distorting signals are presentin the immediate vicinity of the device-probing end (since relativelyunrestricted coupling of signals can occur between the closelyneighboring strips). Similarly, to the extent that the conductorarrangement in Stewart takes the form of a solitary dot in any onequadrant, if the signal condition in any channel is evaluated via thisdot, it may appear to vary due to coupling between tips and due to anymovement of equipment in the vicinity of the channel, particularly sincethis type of conductor arrangement fails to provide adequate constraintof signal ground. That is, the one or more device-probing ends of thenetwork that normally establish a ground return path for thehigh-frequency signal channels of the network by their connection, forexample, with the ground pad or pads of the device under measurement,are afforded no connection sites in the quadrant of the Stewartcheckplate containing the solitary dot. For the same reason, the Stewartsystem is not able to accurately duplicate during the evaluation sessionthe loading conditions that are present during device measurement.

There are additional disadvantages associated with the Stewart procedureinsofar as the probe card is removed in Stewart from its original holderand remounted in a separate stand-alone station before evaluation of theprobe card begins. Although this remounting procedure allows the Stewartevaluation station to process the signals before they enter thecheckplate, such procedure forecloses the possibility of in situmeasurement of the network.

Other systems that have been developed for precisely locating therelative position of the device-probing ends of a measurement networkare shown in Sigler, U.S. Pat. No. 5,065,092 and in Jenkins et al., U.S.Pat. No. 5,198,756. These systems, like that of Stewart, are inadequatefor high-frequency measurement for similar reasons.

In accordance with the foregoing, then, an object of the presentinvention is to provide an improved system for evaluating thehigh-frequency characteristics of a probe measurement network withreference, in particular, to the device-probing ends of such network.

A related object of the present invention is to provide an improvedinterconnect assembly for uniformly transferring high-frequency signalsto and from the device-probing ends of a probe measurement network,particularly when such ends are arranged for the measurement of planarmicroelectronic devices.

BRIEF SUMMARY OF THE INVENTION

The present invention solves the foregoing difficulties by providing animproved assembly and method for evaluating the signal conditions in aprobe measurement network.

In a first aspect of the invention, an improved assembly is provided foruse in evaluating network signal conditions. The assembly includes abase on the upper face of which are located respective first and secondconductive planar probing areas. These areas are in spaced-apartmutually coplanar relationship to each other and are so arranged that afirst and second device probing end of the probe network can besimultaneously placed on the first and second conductive planar probingareas, respectively. The improved assembly further includes a referencejunction and a high-frequency transmission structure connecting thefirst and second probing areas to the reference junction such that foreach position that the ends can occupy while on the corresponding areas,a transmission line of substantially constant high-frequencytransmission characteristic is provided between these ends and thereference junction.

In accordance with the foregoing combination, there will be asubstantially uniform relationship between the entering condition andthe exiting condition of each high-frequency signal that is transmittedbetween the device-probing ends and the reference junction regardless ofwhich probing position on the areas is occupied by the ends during eachtransmission. Hence, if a reference sensing unit, for example, isconnected to the reference junction and the exiting condition of eachsignal is the same as measured at the reference sensing unit, then thisconfirms that the entering condition of each signal transferred to theareas by the ends was likewise the same, irrespective of the probingposition used for each measurement. Conversely, if a reference sourcingunit is connected to the reference junction, so that the enteringcondition of each signal is the same, then the exiting condition of eachsignal that is transferred to the ends by the areas will likewise be thesame irrespective of the probing position used during each transfer.

In accordance with a second aspect of the present invention, an improvedmethod is provided for evaluating the signal conditions in a probemeasurement network of the type having a plurality of separatemeasurement channels, where each channel communicates through acorresponding device-probing end. The method includes providing anassembly which includes a conductive planar probing area on the upperface of a base and a reference junction connected to the probing area bya high-frequency transmission structure. The method further includesplacing the respective device-probing end of a first one of themeasurement channels into contact with the planar probing area,transmitting a high-frequency signal through both the measurementchannel and the reference junction and, thereafter, measuring thesignal. This step is repeated for the other measurement channels and thesignal conditions in the different channels are then evaluated bycomparing the measured signals, where such evaluation is facilitated bymaintaining, via the high-frequency transmission structure, atransmission line of substantially constant high-frequency transmissioncharacteristic between each device-probing end coming into contact withthe planar probing area and the reference junction.

In accordance with the above method, high-frequency signals can beuniformly transferred from the device-probing ends to a referencesensing unit connected to the reference junction, thereby enablingaccurate calibration of the incoming or source channels of the network.Conversely, high-frequency signals can be uniformly transferred to thedevice-probing ends from a reference sourcing unit connected to thereference junction, thereby enabling accurate calibration of theoutgoing or sense channels of the network.

The foregoing and other objectives, features, and advantages of theinvention will be more readily understood upon consideration of thefollowing detailed description of the invention, taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a perspective view of a probe station, in accordance with theprior art, which includes a probe measurement network having a probecard assembly for probing various microelectronic devices on a wafer,also shown.

FIG. 2 is an enlarged cutaway plan view of an individual planar devicelocated on the wafer of FIG. 1, which device is schematicallyrepresented for ease of illustration.

FIG. 3 is a perspective view of an interconnect assembly of a type used,in accordance with the prior art, for evaluating probe measurementnetworks of the type shown in FIG. 1.

FIG. 4 is a perspective view of an exemplary interconnect assembly,constructed in accordance with the present invention, which assembly isintegrated with a probe station, also shown, for enabling rapid andaccurate calibration of the station's measurement network.

FIG. 5 is a sectional view primarily of the exemplary interconnectassembly as taken along lines 5-5 in FIG. 4.

FIG. 6 is an enlarged cutaway plan view of the center portion of theexemplary interconnect assembly as taken within the dashed-line regionindicated by reference number 60 in FIG. 5.

FIG. 7 is an enlarged sectional view of the exemplary interconnectassembly, as taken within the dashed-line region indicated by referencenumber 60 in FIG. 5, together with an enlarged elevational view ofcertain of the device-probing ends of the probe card of FIG. 4 so as toindicate the arrangement of these ends relative to the probing areas ofthe assembly.

FIG. 8 is a sectional view corresponding in viewing angle to the view ofFIG. 5 of a first alternative embodiment of the interconnect assembly.

FIG. 9 is an enlarged sectional view of the first alternative embodimentof FIG. 8, as taken within the dashed-line region indicated by referencenumber 118 in FIG. 8, together with an enlarged elevational view ofcertain of the device-probing ends of the probe card of FIG. 4 so as toindicate the arrangement of these ends relative to the probing areas ofthe assembly.

FIG. 10 corresponds to FIG. 6 except that it shows, in solid-line view,the probing area configuration of a third alternative embodiment of theinterconnect assembly especially suited for the simultaneous measurementof a pair of signal channels and further shows, in dashed-line view,different positions which the device-probing ends can occupy in relationto this probing area configuration.

FIGS. 11 a-d are schematic plan views depicting consecutive positions ofthe device-probing ends of the probe card, represented in dashed-lineview, on the probing areas of the exemplary interconnect assembly ofFIG. 4, represented in solid-line view, during an exemplary probenetwork evaluation session.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

FIG. 4 shows an exemplary interconnect assembly 48 which is constructedin accordance with the present invention and which, in accordance with apreferred method, enables accurate calibration of a multichannel probenetwork 21. The network can include, as shown, a probe card 30 of thetype having a plurality of needle-like probe tips 34, where the lowerextremities of these tips form the device-probing ends of the networkand these ends are so arranged as to match the pad arrangement of aspecified group of microelectronic devices 24, such as those formed onan integrated wafer 22. The assembly 48, in particular, is so configuredas to enable the uniform transfer of high-frequency signals between eachdifferent end and a reference sourcing or sensing unit despite thefragility of the needle-like probe tips 34 and the closely crowdedarrangement of the device-probing ends.

In the exemplary embodiment shown, the reference sourcing or sensingunit will be understood to be detachably connected to the underside ofthe assembly 48 by a high-frequency channel, such as a coaxial cable 49.This reference unit can be provided by the same test instrument 36which, with its different sourcing and sensing units, generates andmonitors the various signals that are present in the network 21, thatis, the signals which are transferred to and from each device 24 duringdevice measurement. In the particular setup shown in FIG. 4, then, theprobe measurement network 21 includes, in addition to the probe card 30,various sources and sensing units within the test instrument as well asthe multiconductor measurement cable 38 that connects the card andinstrument.

FIG. 7 shows an enlarged elevational view of certain ones of theneedle-like probe tips 34 which are included on the probe card 30 shownin FIG. 4. During device measurement, the device-probing ends of thesetips, such as 50 a-b, 52 a-b and 54 a-b, are used to transferhigh-frequency signals to or from the device-under-test. Thesehigh-frequency signals are typically within the range of 100 MHZ to 2GHz. However, the term “high-frequency” is more broadly intended, hereinand in the claims, to denote any frequency within the range from 50 MHZto 65 GHz or above.

For the sake of convenience, it will be assumed that any device-probingend which is identified in the drawings by a reference number startingwith 50, such as ends 50 a and 50 b, corresponds to a source channel ofthe probe measurement network 21, that is, from each such end theincoming signal from a respective sourcing unit in the network 21 isdirectly transferred to a corresponding input pad of thedevice-under-test. Similarly, it will be assumed that any end which isidentified by a reference number starting with 52, such as ends 52 a and52 b, corresponds to a sense channel of the network, that is, theoutgoing signal from a particular output pad of the device-under-test isdirectly transferred to a corresponding one of these ends for subsequenttransmission to a respective sensing unit in the measurement network.Finally, it will be assumed that any end which is identified by areference number starting with 54, such as ends 54 a and 54 b,corresponds to a ground return line for the source and sense channels ofthe network, that is, during device measurement, each such end connectsto a corresponding ground pad of the device-under-test so that awell-constrained ground line is established for each signal channel.This designation of particular ends as corresponding to either source,sense or ground return lines is conventional and is indicated hereinonly to clarify the operation of the exemplary interconnect assembly 48,a portion of which is also shown in FIG. 7 in sectional view.

Referring to FIGS. 4 and 7 together, the exemplary interconnect assembly48 enables uniform transfer of high-frequency signals between thereference channel 49 and each signal-carrying end (e.g., 50 a-b and 52a-b). This, in turn, makes it possible to obtain accurate comparativeinformation about the relative signal conditions in the differentchannels and thus, in accordance with a preferred method describedhereinbelow, enables accurate calibration of the probe measurementnetwork 21. Referring also to FIG. 5, the assembly 48 includes a baseassembly 56 and a movable support assembly 58. The characteristics ofthe base assembly, in particular, enable signal transfer operations tobe performed with substantial uniformity.

The preferred construction of the base assembly 56 is best illustratedin FIG. 7 which, in enlarged sectional view, shows the central region ofthe base assembly as included within the dashed-lined area 60 identifiedin FIG. 5. The base assembly 56 includes a base member 62 which, in thepreferred embodiment shown, constitutes a plate of solid brass. A seriesof concentrically aligned cavities are centrally formed in this plateincluding a lower threaded cavity 64 within which a high-frequencycoaxial adapter 68 is screwably installed. In the preferred embodimentshown, this adaptor is a “sparkplug” type K-connector of the type sold,for example, by Wiltron Company of Morgan Hill, Calif. under Model No.K102F. This adaptor enables detachable connection of different types ofreference units (i.e., of sensing or sourcing type) to the base member62. Such connection can be made, as shown, through a coaxial cable 49,the end of which includes a threaded connector 70 which is suitablydimensioned for attachment to the adapter.

Formed within the brass base member 62 above the lower threaded cavity64 are a lower mounting cavity 72, center cavity 74 and upper mountingcavity 76. A K-connector bead 78 is mounted within the lower mountingcavity 72. This bead, also known in the art as a “glass” bead, is usedin conventional assemblies in connection with an associated fixture inorder to interconnect a K-connector of the type above described with aplanar microstrip line. A bead of suitable type, for example, is sold byWiltron Company under Model No. K100. This bead includes an innerconductor 82 that has a nominal diameter of about 12 mils. In thisapplication, the first or lower end 80 of the inner conductor 82 isinsertably engaged, in a conventional manner, with the tubular centerconductor 84 of the K-connector 68. The second or upper end 86 of theinner conductor 82, which normally extends outwardly from the bead forconnection with a microstrip line, is cut short so that only a smallportion of the inner conductor extends past the surrounding innerdielectric 88 of the bead, as shown. The inner dielectric of the bead ismade of glass for low-loss transmission of high-frequency signals andthe bead further includes an outer conductor or metalized rim 90 thatconcentrically surrounds the inner conductor 82. This rim is soldered tothe lower mounting cavity 72 so that the bead 78 is fully seated withinthis cavity, as shown.

A pocket 92 is drilled into the upper end 86 of the inner conductor 82of the bead and a length of copper wire 94 of 3 mil nominal diameter orother suitable conductor is anchored by its lower end within the pocketby a low temperature solder 95 so that the respective center axes of thewire and the inner conductor are aligned. An annular-shaped glass sleeve96 of 10 mil wide outer diameter is then fitted over the wire, and theouter sides of the sleeve are epoxied to the upper mounting cavity 76 ofthe brass base member 62. Within the center cavity 74 of the basemember, the lower face of the glass sleeve abuts the upper end 86 of theinner conductor 82. A lapping process is used to remove excess materialalong the upper face 104 of the base member so as to present acompletely flat and smooth surface along this face. The upper face ofthe brass base member 62 and the upper end of the wire 94 are goldplated using an electroplating bath while the upper face of the glasssleeve 96 is covered by a mask. Referring also to FIG. 6, in accordancewith these processing steps, the plated surfaces of the wire 94 and thebrass base member 62 form a first or inner planar probing area 98 and asecond or outer planar probing area 100, respectively. As shown in FIG.6, the outer probing area 100 is radially spaced apart from andcompletely surrounds the inner probing area 98, and the exposed face ofthe glass sleeve forms an annular-shaped dielectric area or“transmission window” 102 between these two probing areas.

Referring to FIGS. 6 and 7 together, the inner and outer probing areas98 and 100 are both included on the upper face 104 of the base member62. As used in this context, the term “on” is intended to mean “withinthe outer boundaries of.” It will be seen from FIG. 7 that the inner andouter probing areas 98 and 100 and the dielectric area 102 aresubstantially level with each other so that there is no protruding edgealong the upper face 104 of the base member that could snag and damagethe delicate needle-probe tips 34 as these probe tips are being movedbetween different probing positions on the probing areas.

A high-frequency transmission structure or channel 106 is formed withinthe base member 62 in such a manner as to be integrally connected withthe respective probing areas 98 and 100. In effect, the areas 98 and 100define that section of the transmission structure which adjoins theupper surface 104 of the base member. This transmission structureenables high-frequency signals to travel through the base memberperpendicular to the principal plane of the base member. In thepreferred embodiment shown in FIG. 7, the transmission structure hasinner and outer boundaries where the outer boundary is formed by theupper mounting cavity 76, the center cavity 74 and the inner surface 108of the metalized rim 90. The inner boundary of the transmissionstructure is formed by the respective outer surfaces of the copper wire94 and the inner conductor 82.

The high-frequency transmission structure 106 is connected to thehigh-frequency coaxial adapter 68 at a reference junction 110 (a portionof the inner conductor 82 extends past this reference junction to matewith the tubular center conductor 84 of the coaxial adapter). Thereference junction is suitably configured for connection to thereference sourcing or sensing unit. In particular, the reference unitcan either be connected directly to the reference junction with itsconnector screwed into the lower threaded cavity 64 or, as shown inFIGS. 5 and 7, the reference unit can be connected indirectly to thereference junction by a coaxial adapter 68 and a coaxial cable 49. Theadvantage of using the cable is that the reference junction can, as theneed arises, be flexibly connected to more than one type of referenceunit. Essentially, then, with respect to the exemplary interconnectassembly 48, the reference junction 110 serves as either a presentationnode or a sampling node for reference signals.

As previously noted, FIG. 6 shows a plan view of the inner probing area98 as well as that portion of the outer probing area 100 which iscontained within the dashed-line area 60 indicated in FIG. 5. Theportions of the outer probing area that are not shown in FIG. 6 extendto the outer edges of the base member 62 (FIG. 7). Thus, as indicated inFIG. 4, the outer probing area is considerably larger than the innerprobing area and, indeed, nearly covers the entire upper side of theassembly 48. Referring to FIGS. 6 and 7 together, the inner and outerprobing areas are so arranged in relation to each other than anyneighboring pair of device-probing ends of the probe assembly 30, suchas ends 50 a and 54 a, can be simultaneously placed on the inner andouter probing areas so that one end is positioned on each area. Duringthis placement process, the compatible planar geometry of both the endsand the areas not only serves to reduce wear but also ensures that anysignal exchanges between the probe assembly 30 and the interconnectassembly 48 occur uniformly through the extreme ends of the probeassembly rather than through signal exchange sites on the probe assemblythat are randomly located further up along the probe tips.

Referring to FIG. 7, when the signal-carrying end 50 a and itscorresponding ground return end 54 a are placed on the inner and outerprobing areas 98 and 100, respectively, the transmission structure 106provides a transmission line between these ends and the referencejunction 110. In the preferred embodiment shown, this transmission lineassumes the form of a coaxial-type channel having inner and outerconductive boundaries, as previously noted, where the diameter of theinner and outer boundaries changes in a stepwise manner along the axisof the channel so as to reduce transition discontinuities.

More generally, the term “transmission line” as used herein and in theclaims is intended to denote any signal-guiding structure that includesspaced-apart boundaries, where the boundaries are capable of supportinga high-frequency field so as to enable the propagation of ahigh-frequency signal along the boundaries. These boundaries cancomprise, for example, reflection surfaces between which, at any sectionof the line, there is a predetermined spacing to ensure signalstability. Although the preferred embodiment of the transmissionstructure 72 has boundaries which are formed by certain portions of themetallic surfaces of the brass base member 62, the K-connector bead 78and the copper wire 94, for certain applications it may be preferable toconstruct the transmission structure without any metallic materials. Ifthe signal frequency of the probe network is within the optical portionof the frequency spectrum, for example, it may be preferable toconstruct the transmission structure using only dielectric materials inwhich the boundaries are formed by differently doped regions in likemanner to an optical fiber. Similarly, the term “conductive,” as usedherein and in the claims, is intended to refer generally to the abilityof a certain element to conduct a signal without limitation as towhether or not, for example, the element is a metal.

With respect to the exemplary embodiment shown in FIG. 7, theconfiguration of the transmission structure 106 is such that for eachdifferent position that the ends 50 a and 54 a can occupy when they areplaced on the corresponding areas 98 and 100, the transmission linewhich the structure provides between these ends and the referencejunction 110 has a substantially constant high-frequency transmissioncharacteristic. In particular, the transmission structure is soconfigured that a signal traveling along the structure propagates in adirection perpendicular to the plane of the probing areas rather than ina direction parallel to that plane. As a result, the size of the innerprobing area 98 is not determined by the length of the transmissionstructure and hence can be reduced, as shown, to a dimensionapproximately corresponding to that of each device-probing end. In FIG.7, accordingly, if the inner probing area 98 is lifted straight up inthe direction indicated, so as to come into contact with thedevice-probing end 50 a, regardless of whether this end was initially ina centered position relative to the area, as shown in solid-line view,or was, instead, in a misaligned position 112 off to one extreme edge ofthe area, as indicated in dashed-line view, once the end is actuallypositioned on the area, a signal passing between the end and thereference junction will travel over substantially the same path foreither position. That is, the signal will experience substantially thesame delay, loss and dispersion for each position in accordance with thecharacteristics of the transmission line provided by the structure 106.

Just as it is possible for the respective signal-carrying ends to occupydifferent positions on the inner probing area 98, so it is also possiblefor the respective ground return ends to occupy different positions onthe outer probing area 100. This is best illustrated in FIGS. 11 a-11 dwhich indicate, with directional arrows, how the exemplary interconnectassembly 48 is shifted in mutually perpendicular directions during atypical network evaluation session in order to consecutively positiondifferent ends of the probe array on the inner probing area 98,including the corner ends 50 c-50 f. In FIGS. 11 a-11 d, thedevice-probing ends of the network are represented in dashed-line viewand the number of ends per side is considerably reduced from theiractual number for ease of illustration. Referring to FIG. 11 a, it willbe recognized that when the signal-carrying end 50 c is positioned onthe inner probing area 98, the corresponding ground return end 54 c ispositioned on the outer probing area 100 in a position lying to the“east” of the inner probing area. On the other hand, as shown in FIG. 11b, when the signal-carrying end 50 d is positioned on the inner probingarea because of a shift by the exemplary assembly 48 in the directionindicated in FIG. 11 a, the corresponding ground return end 54 d ispositioned on the outer probing area in a position that now lies to the“north” of the inner probing area. Similarly, as indicated by theposition of end 54 e in FIG. 11 c and by the position of end 54 f inFIG. 11 d, in accordance with the shifting sequence shown, it is alsopossible for the respective ground return ends to occupy positions onthe outer probing area that lie to the west or south of the innerprobing area.

Regardless, however, of whether the ground return end occupies aposition lying to the north, south, east or west of the inner probingarea 98, the corresponding transmission path provided by thetransmission structure 106 is substantially the same for each position.Referring to FIGS. 6 and 7 together, it will be recognized that as longas the subject signal-carrying end remains in its position on the innerprobing area 98, the geometry, and hence circuit characteristic, of theouter probing area 100 will appear substantially identical to thecorresponding ground return end for any angular position which that endcan then occupy. Similarly, subject to the same condition, the geometryof the upper mounting cavity 76, the center cavity 74 and the innersurface 108 of the rim 90 will appear the same to the ground return endwith respect to any angular position of that end due to the angularsymmetry of each of these elements. Hence, the exemplary transmissionstructure 72 provides an omnidirectional transmission line which, inrelation to any pair of corresponding ends, presents a substantiallyuniform transmission characteristic for any angle which those ends canassume while remaining on their corresponding areas.

Referring to FIG. 7, the exemplary transmission structure 106 thereinshown not only provides a stable transmission line between the ends ofthe probing network and the reference junction 110 for differentpositions of the ends on the corresponding areas 98 and 100, but also isso configured that high-frequency signals that are present in theenvironment but that are not the subject of evaluation will, as ageneral matter, be prevented from entering this transmission line. Forexample, the incoming signal that is the subject of evaluation in FIG.7, that is, the signal which is contained in the field supported by ends50 a and 54 a, will have little difficulty passing through the energy“window” that is provided by the dielectric area 102 between the innerprobing area 98 and the outer probing area 100. On the other hand, theincoming signal that is contained in the field supported by ends 50 band 54 a, which signal is not of interest, will be reflected the momentit reaches the plane of the outer probing area 100 in a direction awayfrom the transmission line. In effect, the outer surfaces of the basemember 62 form an electromagnetic shield in relation to the transmissionline that substantially prevents radiation from entering the line fromany source adjacent the substrate's upper face 70 that is other than thedevice-probing end under evaluation.

Referring to FIG. 7, it has now been described how the base assembly 56facilitates uniformity of signal transfer between the device-probingends of the network and the reference junction 110. In particular, ithas been explained how the signals which pass between the ends and thereference junction are substantially unaffected by the type of variationin probing position that is likely to occur as the inner probing area 98is shifted from end-to-end. It has further been explained how the baseassembly 56 rejects high-frequency signals, except from the subjectchannel, so that these signals cannot enter the evaluation path anddistort the signal of interest. Accordingly, at least two distinctaspects about the base assembly 56 facilitate uniformity of signaltransfer, namely its substantial immunity to normal probing positionvariation and its substantial immunity to spurious signals.

It will be recognized that alternative forms of the interconnectassembly 48 are possible. For example, referring to FIGS. 8 and 9, whichcorrespond in viewing angle to FIGS. 5 and 7, respectively, a firstalternative embodiment 114 of the interconnect assembly is shown. Inthis embodiment, the base member 116 constitutes a substrate which isonly about 5 to 25 mils thick with a nominal thickness of about 10 mils.This substrate is preferably made of glass or other hard dielectricmaterial in order to reduce the flow of leakage currents within thesubstrate at higher frequencies. Referring to FIG. 9, which is anenlarged view of the dashed-line area 118 shown in FIG. 8, a first orinner planar probing area 120 and a second or outer planer probing area122 are formed by a metalization process on the upper face 124 of thebase member so that these areas are in mutually coplanar relationshipwith each other. As was the case with the exemplary interconnectassembly 48, the first and second probing areas 120 and 122 define theupper end of a transmission structure 125 that enables high-frequencysignals to travel through the base member perpendicular to the principalplane of such base member. In the first alternative embodiment 114, theinner and outer boundaries of the transmission structure are formed by afirst or inner conductive via 126 and a second or outer conductive via128, respectively, where the outer conductive via is generally annularin shape. Each via 126 or 128 is embedded within the substrate andextends directly beneath the corresponding probing area 120 or 122.

The first alternative embodiment 114 of the interconnect assembly, likethe exemplary interconnect assembly 48, includes a reference junction130. In the first alternative embodiment, this reference junction isdefined by that section of the high frequency transmission structure 125which is contiguous with the lower surface of the substrate 116.

The movable support assembly 131 of the first alternative interconnectassembly 114 includes a horizontal portion 132. A concentrically alignedseries of cavities 134, 136 and 138 are formed in this horizontalportion, and a high-frequency “sparkplug” type connector 140 isscrewably installed into the lowest cavity 134. A protruding portion ofan inner dielectric 142 of this connector is received by the centercavity 136, and an exposed center conductor 144 of the connector passesthrough the upper cavity 138 for electrical connection with the innervia 126. The outer conductive shell 146 of the connector 140, on theother hand, makes electrical connection with the outer via 128 throughthe conductive body of the movable support assembly 131. Solder,conductive epoxy or other electrically conductive joining material isused to permanently bond the center conductor 144 to the inner via andthe conductive body of the movable support assembly 131 to the outervia. These connections ensure continuity in the ground return path forany ground return end positioned on the outer probing area 122 and alsoprovide a well-isolated controlled impedance path for all signalstransmitted between the reference junction 130 and the reference channel49. As with the exemplary interconnect assembly 48, the high-frequencyconnector 140 of the first alternative interconnect assembly 114 can beremoved, and the connector head of a reference unit can be connecteddirectly to the reference junction 130.

Referring to FIG. 7, the outer boundaries of the inner probing area 120and the outer probing area 122 are defined by an inner well 148 and anouter well 150, respectively, that are formed on the upper face 124 ofthe substrate 116. A high-definition masking process and a suitableetching agent, such as hydrofluoric acid, are used to form these wells.As shown in FIG. 7, the wells are separated from each other by a narrowannular ridge 152 of substrate material. Within the substrate 116, innerand outer sloping wall channels 154 and 156 are formed using a laser.These channels define the boundaries of the inner and outer vias 126 and128. In forming the sloping wall channel 154 for the inner via, thetranslucent property of the glass substrate permits the laser beam to beaccurately focused directly opposite and on-center with the well 148that was previously etched for the inner probing area. The inner andouter wells 148 and 150 and the corresponding sloping wall channels 154and 156 are then filled with nickel or some other suitably hard metal(such as tungsten, iridium or rhodium) in such a manner that the metalin the wells is able to fuse with the metal in the channels. Thesloping-wall shape of each channel facilitates this fusing process bymaking it easier for the metal to flow into and completely occupy eachchannel. An alternative approach is to omit the step of forming thewells 148 and 150 and to simply deposit extremely thin layers ofconductive material on the substrate 116 to form the probing areas 120and 122. Under this alternative approach, however, there is asignificant risk that the ends of the tips 34 will punch through thethin probing areas if an excessive amount of contact force is appliedbetween these ends and the areas.

After being formed, the upper surfaces of the inner and outer probingareas 120 and 122 are then lapped until the resulting overall surface iscompletely flat and smooth. As with the exemplary interconnect assembly48, this step ensures that there are no protruding edges left on theupper face of the assembly 114 which could snag and damage the delicateneedle-like tips during repositioning of the assembly. In the embodimentof the first alternative interconnect assembly shown in FIG. 7, themaximum edge-to-edge dimension 157 of the inner probing area 120 isnominally 4 mils while the radially-extending gap 158 between the innerand outer probing areas 120 and 122 is nominally ½ mil across.

From the foregoing description of the first alternative interconnectassembly 114, it will be recognized that the basic components of thisassembly correspond to those of the exemplary interconnect assembly 48,insofar as both assemblies include a base member 62 or 116, first andsecond planar probing areas 98 and 100 or 120 and 122 on the basemember, a transmission structure 106 or 125 extending perpendicular tothe principal plane of the base member and a reference junction 110 or130 that serves as a connection site for the reference unit. It willfurther be recognized that the functional advantages noted above inconnection with the exemplary interconnect assembly 48 apply equally tothe first alternative interconnect assembly 114, that is, the assembly114 is able to uniformly transfer signals despite variation in theposition of the device-probing ends 34 on the probing areas and,furthermore, is able to reject spurious signals that originate outsideof the channel under evaluation.

As shown in FIGS. 7 and 9, both the exemplary interconnect assembly 48and the first alternative interconnect assembly 114 include two probingareas on their respective upper faces consisting of the inner probingarea 98 or 120 and the outer probing area 100 or 122. This configurationis suitable for the closely crowded needle-like tips 34 (FIG. 4) whichare commonly used to test microelectronic devices, particularly sincethe signal field of any one channel in this type of network is supportedbetween a corresponding signal tip and a nearby ground return tip.However, it may be preferable in some applications to have only oneprobing area on the upper face of the assembly (as, for example, whenthe signal field is carried by a single optical fiber). Furthermore, inorder to evaluate some types of probe network conditions, such as thecross-talk between two signal channels of the network, it may bedesirable to have three probing areas on the substrate's upper face.

FIG. 10 shows a second alternative embodiment 160 of the assembly whichincludes three probing areas for use in evaluating network conditionssuch as cross-talk. This embodiment includes a base assembly comprisinga substrate 162 on the upper face of which is a first inner probing area164, a second inner probing area 166 and a third or outer probing area168. If the substrate 162 were to be viewed in section along thereference line 170, the resulting figure would correspond closely toFIG. 9 except that there would be a pair of inner vias in balancedarrangement within the outer via 128 instead of only one inner via 126.

In FIG. 10, the device-probing ends are represented in dashed-line view.With respect to one of the illustrated probing positions, the entireprobing array is depicted in order to show how the square-like probingarray is preferably oriented in relation to the three probing areasduring evaluation of the probing network. The number of ends per side ofthe array has been reduced from their actual number for convenience andclarity.

For purposes of illustration, it will be assumed that the twosignal-carrying ends 50 a and 50 b correspond to the two channels of thenetwork that are the subject of interest and that the correspondingground return end is 54 a. It will be recognized that when these endsare in the center probing position illustrated, that is, when these endsare aligned with reference line 170, then the ends are mispositionedsince end 50 a, corresponding to a signal-carrying channel, properlybelongs on one of the inner or signal probing areas 164 or 166, whileend 54 a, corresponding to a ground return line, properly belongs on theouter or ground probing area 168. It is possible, however, to shift thesubstrate 162 so as to simultaneously position these three ends oncorresponding ones of the areas. That is, the substrate can be shiftedso that the first signal-carrying end 50 a occupies a position on thefirst inner probing area 164, the second signal-carrying end 50 bsimultaneously occupies a position on the second inner probing area 166and the ground return end 54 a simultaneously occupies a position on thethird or outer probing area 168. For example, the substrate can beshifted in the −X and −Y directions indicated in order to repositionthese ends to the illustrated probing position that is in alignment withreference line 172. Alternatively, the substrate can be shifted in the+X and −Y directions indicated in order to reposition these ends to theillustrated probing position that is in alignment with reference line174.

In the example just given, it will be recognized that in order for allthree ends 50 a, 50 b and 54 a to be simultaneously placed on thecorresponding areas identified above, one or the other of the twoillustrated probing positions that is in alignment with reference lines172 or 174 has to be selected. Because of the symmetry of these twoprobing positions with respect to the illustrated center probingposition and also because of the balanced arrangement of the inner viasrelative to the outer via in the underlying transmission structure, atransmission line of substantially constant high-frequency transmissioncharacteristic is presented to ends 50 a, 50 b and 54 a for each probingposition in which the three ends are simultaneously situated on theircorresponding areas. FIG. 10 also illustrates how the probing areaconfiguration provided by the second alternative embodiment 160accommodates the evaluation of cross-talk between two network channelseven when the corresponding ends of the network are spaced widely apart,as is the case with ends 50 g and 50 h.

As explained above, the coaxial adapter 68 or 140 of each interconnectassembly provides a means through which different types of referenceunits can be conveniently connected to the corresponding referencejunction 110 or 130. However, various other types of connections to eachreference junction are also possible. For example, it is possible toadapt each reference junction for direct connection with switches, noisesources, diodes, power sensor elements, couplers, in-line transmissiondevices and various other components. Furthermore, a pair of coaxialadapters instead of only one adapter could be connected to the junction.This last type of connection is the type preferably used, for example,where the interconnect assembly includes a pair of inner probing areas164 and 166, as shown in FIG. 10. As noted above, the second alternativeembodiment 160 shown in FIG. 10 is configured so as to include two innervias, thus providing sufficient attachment sites for the connection oftwo adapters. Such connection permits differential measurements to bemade between two different signal channels of the probing network.

Another variation is to eliminate the movable support assembly 58 or 131so that the base member 62 or 116 is simply placed on the chuck of anyprobing station in the same manner as a wafer. With respect to thisembodiment, any coaxial adapter that is provided on the base memberwould be included on the upper side of such base member (i.e., in aposition that would not interfere with the placement of thedevice-probing ends). To convey the signal between the referencejunction adjacent the lower side of the base member and the adapter onthe upper side, a two-section transmission line may be used, where onesection extends between the reference junction and a point opposite theadapter and the second section, starting from that point, extendsthrough the substrate to the adapter. Still another variation is toconnect a first circuit element to the reference junction on the basisof the characteristics of a second circuit element that is connecteddirectly between the inner and outer probing areas 98 and 100 so that asignal entering the transmission structure 106 will, in effect, see adifferent circuit depending on whether it enters that structure from theupper or lower end. The types of elements that are suitable forconnection directly between the inner and outer probing areas includechip resistors, capacitors and inductors.

Yet another variation is to utilize a connected pair of interconnectassemblies 48 and to support this pair of assemblies so that the spacingbetween the corresponding pair of inner probing areas 98 is adjustableso as to enable probing of the inner probing area of one of theassemblies by a selected one of the “stimulus” probe ends (e.g., 50 b)while concurrently enabling probing of the inner probing area of theother assembly by a designated one of the “response” probe ends (e.g.,52 b) regardless of the spacing that exists between these ends. In thisvariation, the outer probing area 100 surrounding each inner probingarea is reduced in size (e.g., to a nominal radius of about 20 mils) sothat the respective inner probing areas of the corresponding assembliescan be moved adjacent to each other when measurements are to be takenacross nearby probe ends.

In order to ensure that the electrical characteristics of thetransmission channel between the two assemblies is constant for eachselected spacing, the high-frequency adapters 68 of the two assembliesare normally connected together by a short length of flexible coaxialline, although other types of circuit elements can be used as well. Therelative positions of the two assemblies may be adjusted by providing afirst mechanism for adjusting the linear separation between the twoassemblies, a second mechanism for rotating both assemblies in unison,and a third mechanism for effecting X-Y-Z directional movements of thewafer stage that carries both assemblies.

Referring to FIGS. 4 and 5 together, in accordance with a preferredembodiment of the invention, the exemplary interconnect assembly 48hereinabove described is combined with the probe station containing themeasurement network 21 so as to form an integrated self-evaluatingprobing system 176. The exemplary interconnect assembly can either bemounted directly on the wafer-supporting chuck 178, as shown, or can bemounted separately off to one side of the chuck.

Comparing the integrated probing system 176 with the commerciallyavailable wafer-probing station 20 shown in FIG. 1, it will be seen thatthe primary modification made to the pre-existing design in order tomount the interconnect assembly 48 on the chuck 178 has been to cut awaya rectangular corner portion of the chuck so that the movable supportassembly 58 of the interconnect assembly can be conformably fitted tothe square-edged margin 180 thus formed on the chuck. In accordance withthis mounting method, the inner probing area 98 (FIG. 7) of theinterconnect assembly is positioned in close proximity to the wafer 22,and hence only limited-range X-Y-Z movements are needed between thechuck 178 and the probe tips 34 in order to quickly shift these tipsback-and-forth between various device-measuring positions on the wafer22 and various channel-evaluating positions on the interconnect assembly48. With respect to the particular system configuration shown, thismounting method also allows the probe tips to be shifted between theirvarious device-measuring positions and their various channel-evaluatingpositions using the same motorized X-Y-Z positioning table 182 includedin this system for positioning the chuck. However, whether theinterconnect assembly 48 is mounted in adjoining relationship to thewafer-supporting chuck 178, is mounted separately off to one side of thechuck, or is transported to the chuck as the need arises, it isadvantageous when evaluating the probing network 21 to locate theinterconnect assembly in the wafer-probing station, rather than locatingit in a separate evaluation station, in order to permit in situevaluation of the network.

During in situ evaluation of the probe measurement network 21, theoriginal connections that are made in setting up the network aremaintained during evaluation of the network. Accordingly, the results ofthis evaluation will accurately reflect the respective contributionsmade by the original measurement cable 38 and the original testinstrument 36 to the various signal conditions present in the differentchannels of the network. On the other hand, if it were necessary toevaluate the network off-site or in a piecemeal manner, then it would bedifficult to determine the original conditions in the network with thesame degree of accuracy.

For example, if the exemplary interconnect assembly 48 was soconstructed that it could not operate properly unless the signalsentering through the network were first routed through an intermediateprocessing unit, then to use this assembly, it would be necessary tochange the connections of the original measurement network in order toconnect the unit between the network and the assembly. With respect tothe network 21 shown in FIG. 4, for example, the probe card 30 might bedisconnected from the original measurement cable 38 and removed to aseparate evaluation station for connection to the processing unit. Inthis example, because evaluation of the probe card portion of thenetwork would be performed separately from evaluation of the cable andinstrument portions of the network, the resulting procedure would beinherently slower and less accurate than in situ evaluation where theentire network is evaluated at the same time.

With regard to the foregoing, it may be noted that the exemplaryinterconnect assembly 48 does not require a processing unit anywherebetween its probing areas 98 and 100 and the probing network 21 in orderfor it to function properly. If, for example, the source channels of theprobing network 21 are the subject of evaluation, the only connectionneeded to the interconnect assembly is the connection of a referencesensing unit to the reference junction 110 of the assembly. Typically,the original test instrument 36 will include at least one sensing unitthat is not being used for device measurement, which can then beconnected to the reference junction through the reference cable orchannel 49. On the other hand, if the sense channels of the probingnetwork 21 are the subject of evaluation, then the only connectionneeded to the assembly is the connection of a sourcing unit to the samereference junction. Typically, the original test instrument will furtherinclude an unused sourcing unit which can be connected to the referencejunction merely by switching the reference cable 49 to the correspondingport on the instrument, as through a switching device.

As the above examples illustrate, the same piece of equipment 36 that isincluded in the system 176 to process the signals needed for devicemeasurement can also be used, in conjunction with the exemplaryinterconnect assembly 48, to process the signals needed for evaluatingthe system's own probing network, and there is no need, however theassembly is connected, to divide the network 21 into separate parts inorder to use the assembly to evaluate the network. As these two examplesfurther illustrate, the interconnect assembly 48 is bidirectionallyoperable and hence can be used to evaluate each channel of the originalprobing network 21 regardless of whether the channel under evaluation isof source or sense type.

It has now been described how the configuration of the interconnectassembly 48 is compatible with in situ evaluation of the probemeasurement network and how this procedure provides more accurateresults by enabling the channel conditions present during networkevaluation to more closely match those that are present during devicemeasurement. A different aspect of the assembly with the same generaleffect is the ability of the interconnect assembly to emulate certaincharacteristics of a device while it is being probed by the ends of thenetwork.

Referring to FIG. 7, it has been assumed, up to this point, that end 54a provides the ground return path in relation to the signal-carrying end50 a. More generally, however, the ground return path corresponding toend 50 a may be provided by an end that is not immediately adjacent toend 50 a, such as end 54 b. In accordance with this more generalsituation, during device measurement, if end 50 a were to be positionedon an input pad of a device under test, then end 54 b would bepositioned on a ground pad of the same device, so that a continuousground path would be formed as a result of connection between end 54 band the ground pad. This continuous ground path, in turn, would have aneffect on the characteristics of the circuit that is formed through ends50 a and 54 b. This same continuity of ground path is provided, however,by the outer probing area 100, since this area is connected to theground shield of the reference cable 49 through the outer shell of thethreaded connector 70, the outer conductive shell of the coaxial adapter68, and the conductive body of the brass base member 62. In effect, withrespect to the ground return end 54 b, the outer probing area 100appears the same as the ground pad of a device under measurement andthus the channel conditions present during device measurement aresubstantially duplicated during network evaluation.

Referring to FIGS. 7 and 11 a-11 d, with respect to the exemplaryinterconnect assembly 48, any end that is out of contact with the innerprobing area 98 will automatically be positioned in contact with theouter probing area 100. As shown, in FIGS. 11 a-11 d, for example, evenif the end under evaluation is at one of the four extreme corners of theprobe array, as is the case with ends 50 c, d, e and f, if this end ispositioned on the inner probing area 98, then all of the other ends,which include the ground return ends 54 c, d, e and f, are automaticallypositioned simultaneously on the outer probing area 100. In this regard,the inner and outer probing areas 120 and 122 of the first alternativeembodiment 114 are so arranged as to produce the same result. Likewise,with respect to the second alternative embodiment 160 shown in FIG. 10,if two signal-carrying ends, such as 50 a and 50 b, are simultaneouslypositioned so that each is on a corresponding one of the inner probingareas 164 or 166, then the other ones of the ends, such as the groundreturn end 54 a, are automatically positioned simultaneously on theouter probing area 168. Thus, for each embodiment of the interconnectassembly hereinabove described, wherever the ground return end for aparticular source channel happens to be located, the outer probing area100, 122 or 168 will present the same characteristics to that end aswould have been presented by the ground pad of a device during devicemeasurement. It will be noted, on the other hand, that if any end notunder evaluation corresponds to a source channel, such as end 50 b inFIG. 7, the outer probing area will reflect the signal carried by thatend away from the transmission structure 106 or 125, thereby preventingthat signal from entering the evaluation channel and distorting thesignal of interest.

Referring to FIGS. 4 and 5, with the movable support assembly 58 of theinterconnect assembly conformably fitted to the chuck 178 in the cornerposition shown, there remains sufficient area on the chuck to supportthe circular wafer 22. Hence, the respective probing areas of theinterconnect assembly 48 are continuously available for probing by theprobing tips 34 free of any requirement for removal of the wafer fromthe chuck. This, in turn, facilitates rapid back-and-forth shifting ofthe probe tips 34 between various device-testing positions on the waferand various channel-evaluating positions on the interconnect assembly.

Even greater speed in back-and-forth tip positioning is preferablyobtained by using a programmable microprocessor 184 to control themotorized X-Y-Z positioning table 182. This microprocessor is programmedto deliver, in quick succession, a series of control commands to thepositioning table so that predetermined ones of the device-probing endsare positioned by the table, likewise in quick succession, on the innerprobing area 98 during network evaluation.

To evaluate the source channels of the network, for example, thereference junction 110 is connected to a reference sensing unit, and themicroprocessor 183, in response to a user-generated signal, commands thepositioning table to shift so as to move the inner probing area 98 toits starting position (e.g., in contact with end 50 c) as shown in FIG.11 a. Following a preprogrammed set of instructions, the microprocessorthen commands the positioning table 182 to move in specified incrementsalong the +Y direction indicated so that, ultimately, each end alongthis direction that corresponds to a source channel, such as ends 50 cand 50 d, is successively positioned on the inner probing area 98.Referring to FIG. 11 b, without interruption, the microprocessor nextcommands the positioning table to move by specified increments in the −Xdirection indicated so that each end along this direction thatcorresponds to a source channel, including ends 50 d and 50 e, issuccessively brought into contact with the inner probing area 98.Referring to FIGS. 11 c and 11 d, the same procedure is repeated in the−Y direction and the +X direction, thus successively positioning each ofthe ends in the probing array that corresponds to a source channel intocontact with the inner probing area.

After the reference junction 110 has been reconnected to a referencesourcing unit, generally the same sequence is followed in order toevaluate the sensing channels of the measurement network. Starting fromthe position shown in FIG. 11 a, for example, the microprocessor 184begins the sequence by commanding the positioning table 182 to move byspecified increments in the +Y direction indicated, so that each endalong that direction that corresponds to a sense channel is successivelypositioned on the inner probing area, including ends 52 c and 52 d. Thistime, ends 50 c and 50 d, which correspond to source channels, arepassed over. The remainder of the sequence proceeds in similar mannerwith respect to the other directions.

It is also possible to combine the two approaches just described, thatis, the microprocessor 184 can be programmed so that it will instructthe positioning table 182 to successively shift the inner probing area98 into contact with each end that generally corresponds to a signalchannel. Depending, then, on whether the end specifically corresponds toa source channel or to a sense channel, a switching device that isoperated by a control line from the microprocessor will automaticallyconnect the reference junction 110 to either a sensing unit or asourcing unit on the test instrument 36.

With respect to the interconnect assembly 48, a primary advantage tousing the same probing area 98 for evaluating each signal channel isthat if there are any differences detected between the signals in thedifferent channels, these differences can be attributed directly to thesignal channels themselves without further investigation required intothe extent to which these differences might be based on differences inevaluation path. It may also be noted that while the primary anticipateduse of the exemplary interconnect assembly 48 is for the comparativeevaluation of probe measurement channels, the assembly can also be usedfor those situations where quick verification is needed of the qualityof signal at a particular end while device measurement is in progress.

Referring to FIG. 4, damage to the needle-like probe tips 34 couldeasily result if these delicate tips were allowed to become snagged onany portion of the exemplary assembly 48 during rapid repositioning ofthe tips by the automated positioning mechanism just described. As notedabove, the inner probing area 98, the dielectric area 102 and the outerprobing area 100 are substantially level with each other so that thereis no protruding shoulder along the upper face of the assembly whichcould snag and damage the tips.

Referring to FIG. 5, the movable support assembly 58 of the interconnectassembly is provided with a vertical adjustment knob 186 that enablesadjustable positioning of the height of the base member 62 relative tothe wafer-supporting chuck 178 so that the respective probing areas 98and 100 are arranged in adjustable parallel relationship to theimaginary plane defined by the upper surface of the chuck. With thismechanism, then, it is possible to shift the probing areas to a positionthat is in coplanar relationship to the pads on each device 24 of thewafer, irrespective of the particular thickness of the wafer, thusenabling safe back-and-forth shifting of the ends between these pads andthe areas. Referring to FIG. 5, the movable support assembly 58 definesa threadless hole 188 through which the shank of the knob bolt isinserted in order to engage the block while the end threads of thisshank engage an internally threaded bore 190 on the underside of thechuck 178. Hence, by turning the knob one way or the other, theinterconnect assembly 48 can be raised or lowered relative to the chuck.

Referring to FIG. 4, it has been described hereinabove how the exemplaryinterconnect assembly 48 can be used for evaluating the signalconditions in a probe measurement network 21 of the type suitable forprobing of planar microelectronic devices. More specifically, theinterconnect assembly can be used not just to evaluate the network butalso to calibrate the network in accordance with a preferred method thatwill now be described. The object of this calibration can includenormalizing the signal conditions in the network in relation to thedevice-probing ends so that when measurements of a device are madethrough these ends, any channel-to-channel differences which are thendetected are attributable solely to the characteristics of the device.

To explain this object further, typically at least some of thedevice-probing ends that correspond to source channels in the network 21provide different incoming signals to the corresponding input pads ofwhichever device 24 is under test because each incoming signal isnormally transmitted from the port of a different data generating orother sourcing unit within the test instrument 36 and travels along adifferent transmission path (e.g., through a different conductor in themeasurement channel 38 and a different conductor in the probe card 30)to reach the corresponding device-probing end. Accordingly, unless thedifferences between these incoming signals are somehow compensated for,what is measured at the respective output pads of the device reflectsnot only the characteristics of the device itself but also thesechannel-to-channel differences in the incoming signals from themeasurement network.

To eliminate the effect of these channel-to-channel differences, inaccordance with a preferred calibration method, a reference sensing uniton the test instrument 36, such as a spare logic analyzing unit, isconnected to the interconnect assembly 48 through the reference cable orchannel 49. Referring to FIG. 7, with respect, for example, to the twosource channels of the network that correspond to ends 50 a and 50 b,the preferred calibration method includes positioning end 50 a intocontact with the inner probing area 98 and transmitting an incomingsignal through the corresponding source channel. This signal enters thetransmission structure 106 and, from there, is transmitted through thereference junction 110 and along the reference cable 49 to the logicanalyzing unit of the test instrument. This step is then repeated withrespect to end 50 b. That is, the exemplary assembly 48 is preferablyshifted by the automatic positioning mechanism described above so as toposition the end 50 b into contact with the inner probing area 98 and asecond incoming signal is then transmitted through the correspondingsource channel. This second signal, like the first signal from end 50 a,enters the transmission structure 106 and follows the same evaluationpath through the reference junction 110 to the same logic analyzingunit.

It has heretofore been described how the transmission structure 106provides a transmission line of substantially constant high-frequencytransmission characteristic between each end coming into contact withthe inner probing area 98 and the reference junction 110. Accordingly,the characteristics of the entire evaluation path between each end andthe logic analyzing unit are substantially the same for each signal.This means, in turn, that in the example just given, after the first andsecond incoming signals are measured at the logic analyzing unit, if acomparison of the resulting measured values indicates that there is adifference between the two signals, then this same differencecorrespondingly exists in reference to the device-probing ends 50 a and50 b. To compensate for this difference, either a computational approachcan be used (under which, for example, a suitable numerical offset isadded to the readings that are taken through each different sourcechannel) or the probing network 21 itself can be adjusted or altered(this could include, for example, automatically tuning each datagenerating unit of the test instrument 36 until the incoming signals aresubstantially identical in reference to the reference sensing unit).

It may be noted that the exemplary calibration procedure just describedcan be used to correct for the effects of various types of signaldifferences, including differences in phase delay, differences in signallevel, differences in noise level and so on. Of course, the type ofparameter that is under evaluation will determine the type of sensingunit required. If, for example, the object is to normalize noise levelsin the different channels, a noise meter unit or other like sensing unitshould be used instead of a logic analyzing unit.

In the example just given, it was described how the ability of thetransmission structure 106 to uniformly transfer signals between theprobing area 98 and the reference junction 110 makes it possible toaccurately calibrate the source channels of the network and, inparticular, makes it possible to adjust the conditions of the incomingsignals so that these signals are identical to each other when theyreach the device-probing ends of the network. However, because theexemplary interconnect assembly 48 is bidirectionally operable, it isalso possible to use the assembly for accurately calibrating the sensechannels of the probing network.

Calibration of the sense channels is normally necessary becausetypically at least some of the sense channels provide conditions for thesignals during transmission and measurement that are different thanthose provided in other ones of the channels. That is, each outgoingsignal from a device-under-test, upon entering the correspondingdevice-probing end of the network, travels along a differenttransmission path (corresponding to a different conductor in the probecard 30 and a different conductor in the measurement channel 38) and ismeasured by a different sensing unit in the test instrument 36, whichunit has its own individual response characteristic. The object ofcalibration with respect to the source channels of the network, then, isto compensate for these channel-to-channel transmission and measurementdifferences so that if identical outgoing signals are presented, forexample, to the sense channels by the device-under-test, then thiscondition is directly and accurately detected by the test instrument.

In accordance with a preferred method of calibrating the sense channels,a reference sourcing unit on the test instrument 36, such as a sparedata generating unit, is connected to the interconnect assembly 48through the reference cable or channel 49. Referring to FIG. 7, withrespect, for example, to the two sense channels that correspond to ends52 a and 52 b, the preferred calibration method includes positioning end52 a into contact with the inner probing area 98 and transmitting areference signal from the reference sourcing unit. This outgoing signalis transmitted through the reference junction 110 and then through thesource channel corresponding to end 52 a until it reaches thecorresponding sensing or logic analyzing unit of the test instrument,where it is measured. Next, end 52 b is positioned into contact with theinner probing area 98 by the automated positioning mechanism abovedescribed. A second reference signal identical to the first istransmitted from the reference sourcing unit and, like the first signal,passes through the reference junction and then through the sense channelcorresponding to end 52 b until it reaches the respective sensing orlogic analyzing unit that corresponds to this second channel, where itis measured.

Because of the ability of the transmission structure 106 to uniformlytransfer signals between the reference junction 110 and each end cominginto contact with the inner probing area 98, the outgoing signal that ispresented to each end 52 a and 52 b will be substantially identical.Accordingly, if the signal readings of the two sensing units thatcorrespond to ends 52 a and 52 b are compared and any difference isfound to exist between these readings, then this indicates that thereare different signal conditions in the two sense channels thatcorrespond to ends 52 a and 52 b.

Should different signal conditions be detected in the sense channels ofthe network, in order to calibrate these channels, eithercomputational-type operations can be used (such as the addition of asuitable numerical offset to the readings of each sense channel) or thenetwork can be adjusted or altered until each sense channel respondsidentically to the same reference signal (this could be achieved, forexample, by automatically tuning the sensing unit of each sense channeluntil each unit responds equally to the signal from the referencesourcing unit).

Although the exemplary construction of the interconnect assembly 48, andits preferred method of use, have now been described, it will berecognized that alternative constructions and uses are possible withoutdeparting substantially from the broader principles of the presentinvention. For example, as noted above, the interconnect assembly,instead of being mounted to a corner of the probe station chuck, couldtake the form of a wafer-like device that could be readily transportedfrom chuck-to-chuck and which could be held, on each chuck, by the samevacuum lock used for holding wafers. Also, as noted above, differenttypes of reference sourcing and sensing units can be connected indifferent ways to the reference junction of the interconnect assemblydepending on what types of conditions are being evaluated. In additionto these and other variations described above, further variations willbe evident to those of ordinary skill in the art from the foregoingdescription.

The terms and expressions which have been employed in the foregoingspecification are used therein as terms of description and not oflimitation, and there is no intention, in the use of such terms andexpressions, of excluding equivalents of the features shown anddescribed or portions thereof, it being recognized that the scope of theinvention is defined and limited only by the claims which follow.

1. A method for evaluating signal conditions in a probe measurementnetwork of the type having a plurality of separate measurement channels,each of said measurement channels communicating through a correspondingdevice-probing end included on a probe tip array, said methodcomprising: (a) providing an interconnect assembly including a basehaving an upper face, a first conductive planar probing area on saidupper face, and a reference junction, said reference junction beingconnected to said first conductive planar probing area by ahigh-frequency transmission structure including a transmission line; (b)placing the respective device-probing end of a first of said measurementchannels into contact with said first planar probing area, transmittinga high-frequency signal through said first one of said measurementchannels, said transmission line, and said reference junction and,thereafter, measuring said signal; (c) repeating step (b) for another ofsaid plurality of measurement channels; and (d) evaluating the relativesignal conditions in different ones of said measurement channels bycomparing the measured signals.
 2. The method of claim 1 includingconnecting a reference sensing unit to said reference junction,transmitting each signal from a different sourcing unit within saidprobe measurement network and measuring each signal at said referencesensing unit.
 3. The method of claim 2 including adjusting said measuredsignals until said measured signals are substantially identical.
 4. Themethod of claim 2 including connecting a reference sourcing unit to saidreference junction, transmitting each signal from said referencesourcing unit and measuring each signal at a different sensing unitwithin said probe measurement network.
 5. The method of claim 4 furtherincluding adjusting each sensing unit until said measured signals aresubstantially identical.
 6. The method of claim 1 including transmittingsaid signal through said first one of said measurement channels beforetransmitting said signal through said reference junction.
 7. The methodof claim 1 including calibrating said probe measurement network byadjusting said measured signals.
 8. The method of claim 1 includingcalibrating said probe measurement network by computationallycompensating for the differences between said measured signals.
 9. Themethod of claim 1 including, in step (a), providing a second conductiveplanar probing area on said upper face in spaced-apart adjacentrelationship to the first said conductive planar probing area, andfurther including, in step (b), simultaneously placing each of thedevice-probing ends that are out of contact with said first conductiveplanar probing area into contact with said second conductive planarprobing area.
 10. The method of claim I including the further steps ofproviding an X-Y-Z positioning mechanism, bringing said device-probingends into contact with different devices at selected times by means ofsaid X-Y-Z positioning mechanism and at different selected times alsobringing selected ones of said device-probing ends into contact withsaid conductive planar probing area by using said X-Y-Z positioningmechanism.
 11. The method of claim 10 including transmitting a secondhigh-frequency signal through a second one of said measurement channels,said transmission line, and said reference junction.
 12. A method forevaluating signal conditions in a probe measurement network of the typehaving a plurality of separate measurement channels, each of saidmeasurement channels communicating through a correspondingdevice-probing end included on a probe tip array, said methodcomprising: (a) providing an interconnect assembly including a basehaving an upper face, respective first, second, and third conductiveplanar probing areas on said upper face, and a reference junction, saidreference junction being connected to said first conductive planarprobing areas by a high-frequency transmission structure including atransmission line; (b) placing the respective device-probing ends of afirst one and a second one of said measurement channels respectivelyinto contact with said first and second planar probing areas,transmitting a respective high-frequency signal through each of saidfirst and second ones of said measurement channels, said transmissionline, and said reference junction, and thereafter measuring each of saidrespective signals; (c) consecutively repeating step (b) for other onesof said plurality of measurement channels; and (d) evaluating therelative signal conditions in different ones of said measurementchannels by comparing the measured signals.
 13. The method of claim 12including the further step of providing a substantially constanthigh-frequency transmission characteristic in said transmission line.